Liquid crystalline compound, medium and display

ABSTRACT

The invention relates to a liquid crystalline compound of the formula I  
                 
 
     wherein X 1  and R 1  are defined as in claim 1. The invention further relates to a liquid crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy containing one or more compounds of formula I and its use in electrooptical displays and projection systems, in particular reflective displays and displays based on a birefringence effect, such as OCB displays.

[0001] The invention relates to a liquid crystalline compound,especially suitable as a component of a liquid crystalline medium withpositive dielectric anisotropy and high optical anisotropy. Furthermore,the invention also relates to such a liquid crystalline medium and itsuse in electrooptical displays and projection systems, in particularreflective displays, LCoS™ displays and displays based on abirefringence effect, such as OCB displays.

[0002] Displays of the OCB (optically compensated bend) are based on abirefringence effect and comprise a liquid crystal layer with a bendstructure. The bend cell, also known as pi-cell, was first proposed byP. Bos et al., SID 83 Digest, 30 (1983) for an electrically controllablehalf-wave plate, whereas the OCB mode for displays was described by Y.Yamaguchi, T. Miyashita and T. Uchida, SID 93 Digest, 277 (1993),followed by papers of T. Miyashita et al. in, inter alia, Proc.Eurodisplay, 149 (1993), J. Appl. Phys. 34, L177 (1995), SID 95 Digest,797 (1995), and C. -L. Kuo et al., SID 94 Digest, 927 (1994). An OCBcell comprises a liquid crystal cell with bend alignment and a liquidcrystal medium with positive Δε. Furthermore, OCB displays as reportedin the above papers comprise one or more birefringent opticalretardation films to eliminate light leakage by the bend cell in theblack state. OCB displays bear several advantages like for example awider viewing angle and shorter switching times than conventionaldisplays based on twisted nematic (TN) cells.

[0003] The above mentioned papers have shown that liquid-crystallinephases must have high values for the optical anisotropy Δn and arelatively high positive value for the dielectric anisotropy Δε, andpreferably should have rather low values for the ratio between theelastic constants K₃₃/K₁₁ and for the viscosity, in order to be usablefor high-information display elements based on the OCB effect. Theindustrial application of the OCB effect in electro-optical displayelements requires LC phases which must satisfy a multiplicity ofrequirements. Particularly important here are chemical resistance tomoisture, air and physical effects such as heat, radiation in theinfra-red, visible and ultra-violet regions and direct and alternatingelectrical fields. Furthermore, LC phases which can be used industriallyneed a liquid-crystalline mesophase in a suitable temperature range, arelatively high birefringence, a positive dielectric anisotropy and alow viscosity.

[0004] LCoS™ (Liquid Crystal on Silicon) displays are known in prior artand are available from Three-Five Systems Inc. (Tempe, Ariz., USA).LCoS™ microdisplays are reflective displays that typically comprise aliquid crystal layer with twisted nematic structure sandwiched by asilicon backplane and a cover glass. The silicon backplane is an arrayof pixels, each of which has a mirrored surface which is at the sametime a conductor. Each pixel comprises a stationary mirror covered by anactive liquid crystal layer with twisted nematic orientation that can beswitched into homeotropic orientation by application of a voltage. LCoS™microdisplays are small with a diagonal of typically less than 1.0″,however, they enable high resolution from ¼ VGA (78 thousand pixels) toUXGA+ (over 2 million pixels).

[0005] Due to the small pixel size LCoS™ displays also have a very thincell thickness, which is typically about 1 micron. Therefore,liquid-crystalline phases used in these displays must in particular havehigh values for the optical anisotropy Δn, in contrast to conventionalreflective type LC displays, which usually require LC phases with lowΔn. At the same time, high reliability against UV light is essential dueto the backlight system of LCoS™ displays and projection displays ingeneral.

[0006] None of the series of compounds having a liquid-crystallinemesophase which have been disclosed hitherto includes a single compoundwhich meets all these requirements. Generally, therefore, mixtures offrom two to 25, preferably from three to 18, compounds are prepared togive substances which can be used as LC phases. However, ideal phasescannot easily be produced in this way, since liquid-crystal materialshaving at the same time high birefringence and low viscosity werehitherto not available.

[0007] OCB mode and LCoS™ displays can be operated as matrix displays.Matrix liquid-crystal displays (MLC displays) are known. Examples ofnonlinear elements which can be used to individually switch theindividual pixels are active elements (i.e. transistors). This is thenreferred to as an “active matrix”, and a differentiation can be madebetween two types:

[0008] 1. MOS (metal oxide semiconductor) transistors on silicon wafersas substrate,

[0009] 2. Thin-film transistors (TFT) on a glass plate as substrate.

[0010] In the case of type 1, the electro-optical effect used is usuallydynamic scattering or the guest-host effect. The use of monocrystallinesilicon as substrate material restricts the display size, since even themodular assembly of various part-displays results in problems at thejoints.

[0011] In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semi-conductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. Intensive research efforts are being made worldwidein the latter technology.

[0012] The TFT matrix is applied to the inside of one glass plate of thedisplay, while the inside of the other glass plate carries thetransparent counterelectrode. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fullycolour-compatible image displays, in which a mosaic of red, green andblue filters is arranged in such a manner that each filter element islocated opposite a switchable pixel.

[0013] The TFT displays disclosed hitherto usually operate as TN cellswith crossed polarizers in transmitted light and are illuminated fromthe back. In case of OCB mode displays, however, reflective displayshave also been proposed by T. Uchida, T. Ishinabe and M. Suzuki in SID96 Digest, 618 (1996).

[0014] The term MLC display here covers any matrix display containingintegrated nonlinear elements, i.e. in addition to the active matrix,also displays containing passive elements such as varistors or diodes(MIM=metal-insulator-metal).

[0015] MLC displays of this type are particularly suitable for TVapplications or for high-information displays in automobile or aircraftconstruction. In addition to problems with respect to the angledependence of the contrast and the response times, difficulties occur inMLC displays due to inadequate resistivity of the liquid-crystalmixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E.,SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay84, September 1984: A 210-288 Matrix LCD Controlled by Double StageDiode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84,September 1984: Design of Thin Film Transistors for Matrix Addressing ofTelevision Liquid Crystal Displays, p. 145 ff, Paris]. As theresistivity decreases, the contrast of an MLC display worsens. Since theresistivity of the liquid-crystal mixture generally decreases over thelife of an MLC display due to interaction with the internal surfaces ofthe display, a high (initial) resistance is very important for displayswhich must have acceptable resistance values over a long operatingperiod. Furthermore MLC displays, especially for outdoor use, areexposed to UV-radiation by the sunlight, a situation similar to theinfluence of the backlight in projection and LCoS™ displays. Compoundsof the liquid crystalline medium, especially tolane derivatives, whichare used to achieve a high optical anisotropy, may decompose underintensive UV radiation, also leading to a decrease of the resistivity ofthe medium.

[0016] The disadvantage of the MLC-TN displays disclosed hitherto is dueto their comparatively low contrast, the relatively high viewing angledependence and the difficulty of producing grey shades in thesedisplays. Thus, there continues to be a great demand for MLC displays,in particular projection displays, displays based on a birefringenceeffect, such as OCB displays, and TFT-displays for outdoor use havingvery high resistivity at the same time as a wide operating temperaturerange, short response times and low threshold voltage, with the aid ofwhich various grey shades can be produced. Furthermore, there is a greatdemand for liquid crystalline media for these kind of displays whichexhibit at the same time low viscosities, high birefringence, relativelyhigh positive dielectric anisotropy and a high UV stability.

[0017] The invention thus provides a highly polar compound whichexhibits a high optical anisotropy, a very high clearing point und UVstability and which is therefore especially suitable as a component ofliquid crystalline media in MLC displays.

[0018] The invention further provides a liquid crystalline medium, whichovercomes the above mentioned disadvantages, at least in part, and atthe same time has a high optical and dielectrical anisotropy, a highclearing temperature and shows a good UV stability.

[0019] Furthermore, the invention provides an electrooptical liquidcrystal display, in particular an active matrix display, a reflectivedisplay and a display of a projection, LCoS™ or OCB mode, which do nothave the above mentioned disadvantages, or do so only to a reducedextent.

[0020] The invention moreover provides cholesteric liquid crystal mediawith a high optical anisotropy and with a good UV stability and acholesteric liquid crystal display.

[0021] Thus, the invention provides a liquid crystalline compound of theformula I

[0022] wherein

[0023] X¹ is OCF₃ and

[0024] R¹ is alkyl with 1 to 20 C atoms.

[0025] The compounds of the formula I exhibit

[0026] a high value of the birefringence,

[0027] a very high positive dielectric anisotropy,

[0028] a very high clearing point,

[0029] a low rotational viscosity,

[0030] a good low temperature stability and

[0031] a high UV stability.

[0032] Furthermore, the invention provides a liquid-crystalline mediumbased on a mixture of polar compounds of positive dielectric anisotropycontaining one or more, i.e., at least one, compound of formula I.

[0033] The liquid crystalline medium according to the present inventionis characterized in that it exhibits

[0034] a high value of the birefringence,

[0035] a high positive dielectric anisotropy,

[0036] a broad nematic phase range, especially a high clearingtemperature,

[0037] a low viscosity, and

[0038] a high UV stability.

[0039] Therefore, the inventive liquid crystalline medium is especiallysuited for electrooptical displays for outdoor usage allowing high UVradiation and a broad working temperature range, especially up to hightemperatures. Furthermore, electrooptical liquid crystal displays with astrong illumination light, like projection displays, e.g. of the LCoS™mode, may benefit from the use of the liquid crystalline mediumaccording to the invention, by a longer lifetime due to the high UVstability. In addition when using the inventive liquid crystal medium inan electrooptical display it is possible to achieve small values of theresponse time, a low driving voltage, a satisfying grey scaleperformance, a wide viewing angle and a high contrast.

[0040] Cholesteric liquid crystal media comprising as a nematiccomponent a liquid crystal medium according to the present invention andas optically active component one or more chiral dopants are alsoprovided by the present invention. The above mentioned advantageousproperties of the inventive liquid crystalline medium, especially thehigh optical birefrigence and good UV stability, are also advantageousfor cholesteric applications. The invention thus also providescholesteric liquid crystal displays, in particular SSCT- andPSCT-displays, comprising a cholesteric liquid crystal medium asdescribed above.

[0041] The invention thus relates to a liquid crystalline compound ofthe formula I as defined above and below.

[0042] Furthermore the invention relates to a liquid-crystalline mediumbased on a mixture of polar compounds having positive dielectricanisotropy, characterized in that it contains one or more compounds offormula I as defined above and below.

[0043] According to the DE 199 19 348 A1, EP 0 439 089 A1 and WO01/12751 the generic formula I*

[0044] wherein R is H, an alkyl or alkenyl radical having 1 to 15C-atoms which may substituted and wherein one or more CH₂ groups may bereplaced by various groups, L is H or F and Y is F, Cl, halogenatedalkyl, alkenyl, alkenyloxy, alkoxyalkyl or alkoxy, is known andcompounds of this formula I* are proposed as co-components of liquidcrystalline mixtures. But compounds of the formula I according to thisinvention are not disclosed.

[0045] Furthermore, the JP 09-030996 describes fluorinated p-terphenylderivatives.

[0046] In the following preferred derivatives of the compound of theformula I which are also preferred components of the liquid crystallinemedium according to the invention are given. Thus, a preferred liquidcrystalline medium according to the invention contains one or more ofthe preferred compounds of the formula I as specified below.

[0047] Preferred compounds of formula I are those wherein R¹ is alkylwith 1 to 8 C atoms. Very preferably R¹ is a straight chain alkylradical and thus R¹ is methyl, ethyl, propyl, butyl, pentyl, hexyl,heptyl or octyl, in particular ethyl, propyl or butyl.

[0048] Compounds of the formulae Ia and/or Ib are very preferred:

[0049] wherein alkyl is an alkyl group with 1 to 8 C atoms.

[0050] The compounds of the formula I are prepared using methods knownto the person skilled in the art. The following reaction schemeillustrates an advantageous synthesis of the inventive compounds.

[0051] In the following preferred liquid crystalline mixtures accordingto the invention are specified.

[0052] Liquid crystalline mixtures with a high optical anisotropy valueΔn≧0.16, especially Δn≧0.17, in particular Δn≧0.18 are preferred. Evenoptical anisotropy values of Δn≧0.19 can be achieved with inventivemixtures.

[0053] The liquid crystalline medium preferably in addition comprisesone or more terphenyl compounds of formula IIa

[0054] wherein

[0055] R⁰ is alkyl, alkoxy, fluoroalkyl, up to perfluoroalkyl, alkenylor oxaalkenyl with up to 9 C atoms and

[0056] X⁰ is F, Cl, CF₃, OCF₃, OCHF₂, fluoroalkyl or fluoroalkoxy up toperfluoroalkyl or alkoxy, with up to 7 carbon atoms.

[0057] R⁰ is preferably an alkyl group with 1 to 8 C-atoms. Preferredmeanings of X⁰ are F, CF₃, OCHF₂ and OCF₃.

[0058] Thus, a particularly preferred liquid crystalline mediumaccording to the invention comprises one or more of the derivatives ofthe formula IIa.1

[0059] wherein alkyl is an alkyl group with 1 to 8 C-atoms.

[0060] Using components of the formula IIa, especially IIa.1, liquidcrystal mixtures with a very high dielectrical anisotropy Δε, especiallyΔε≧12.0, and a very high ε_(∥) value (dielectric constant in theparallel direction to the molecular axis) are obtainable.

[0061] In addition the liquid crystalline medium preferably comprisesone or more terphenyl compounds of formula IIb

[0062] wherein

[0063] R⁰ is alkyl, alkoxy, fluoroalkyl, up to perfluoroalkyl, alkenylor oxaalkenyl with up to 9 C atoms,

[0064] X⁰ is F, Cl, CF₃, OCF₃, OCHF₂, fluoroalkyl or fluoroalkoxy up toperfluoroalkyl or alkoxy, with up to 7 carbon atoms, and

[0065] Y¹ and Y² are each independently of one another H or F.

[0066] R⁰ is preferably an alkyl group with 1 to 8 C-atoms. X⁰ is verypreferably Cl.

[0067] Thus, a particularly preferred liquid crystalline mediumaccording to the invention comprises one or more of the derivatives ofthe formula IIb.1

[0068] wherein alkyl is an alkyl group with 1 to 8 C-atoms.

[0069] Using components of the formula lib, especially IIb.1, liquidcrystal mixtures with a very high optical anisotropy Δn, especiallyΔn≧0.19, are obtainable.

[0070] The liquid crystal mixture preferably comprises in addition oneor more polar compounds selected from the formulae III to VIII

[0071] wherein

[0072] R⁰ is alkyl, alkoxy, fluoroalkyl up to perfluoroalkyl, alkenyl oroxaalkenyl with up to 9 C atoms,

[0073] Z¹ is CF₂O, C₂F₄ or a single bond,

[0074] Z² is CF₂O, C₂F₄ or C₂H₄,

[0075] X⁰ is F, Cl, CF₃, OCF₃, OCHF₂, fluoroalkyl or fluoroalkoxy up toperfluoroalkyl or perfluoroalkoxy, with up to 7 carbon atoms,

[0076] Y¹ to Y⁴ are independently of each other H or F, and

[0077] r is 0 or 1.

[0078] The compounds of formula III are preferably selected from thefollowing formulae:

[0079] wherein R⁰ and X⁰ have the meanings given above, R⁰ is preferablyn-alkyl with 1 to 8 C atoms or alkenyl with 2 to 7 C atoms and X⁰ ispreferably F, Cl, CF₃, OCF₃ or OCHF₂, in particular F or OCF₃.

[0080] Particularly preferably the liquid crystal mixture comprises oneor more compounds of formula IIIa1

[0081] wherein alkenyl is vinyl, 1E-propenyl, 1E-butenyl, 3E-butenyl or3E-pentenyl, in particular 3E-butenyl or 3E-pentenyl, in particularvinyl.

[0082] The compounds of formula IV are preferably selected from thefollowing formulae

[0083] wherein R⁰ and X⁰ have the meanings given above, R⁰ is preferablyn-alkyl with 1 to 8 C atoms or alkenyl with 2 to 7 C atoms, and X⁰ ispreferably F, Cl, CF₃, OCF₃ or OCHF₂, in particular F or OCF₃.

[0084] Particularly preferred are compounds of formula IVa, IVb and IVc,in particular wherein X⁰ is F. Further, compounds of formula IVf arepreferred.

[0085] The compounds of formula VI are preferably selected from thefollowing formulae

[0086] wherein R⁰ and X⁰ have the meanings given above, R⁰ is preferablyn-alkyl with 1 to 8 C atoms or alkenyl with 2 to 7 C atoms, and X⁰ ispreferably F, Cl, CF₃, OCF₃ or OCHF₂, in particular F or OCF₃.

[0087] Particularly preferred are compounds of formula VIa, VIb and VIc.

[0088] The compounds of formula VII are preferably selected from thefollowing formulae

[0089] wherein R⁰ and X⁰ have the meanings given above, R⁰ is preferablyn-alkyl with 1 to 8 C atoms or alkenyl with 2 to 7 C atoms, and X⁰ ispreferably F, Cl, CF₃, OCF₃ or OCHF₂, in particular F or OCF₃.

[0090] Particularly preferred are compounds of formula VIIa and VIIb, inparticular those wherein X⁰ is F.

[0091] The liquid crystal mixture preferably comprises in addition oneor more four-ring compounds selected from the formulae IX to XVI.

[0092] wherein R⁰ and X⁰ have the meanings given above and Y¹, Y², Y³,Y⁴ and Y⁵ are independently of each other H or F with the proviso thatregarding formula XVI Y¹, Y² and Y³ are not F and Y⁴ and Y⁵ are not H atthe same time. Thus, the formula XVI does not encompass compounds of theformula I. R⁰ is preferably n-alkyl with 1 to 8 C atoms or alkenyl with2 to 7 C atoms. X⁰ is preferably F, Cl, CF₃, OCF₃ or OCHF₂, inparticular F or OCF₃. Y⁴ and Y⁵ are preferably H.

[0093] Preferred compounds of the formula IX are those of the formulaIXa

[0094] wherein R⁰ and X⁰ have the meanings given above. X⁰ is preferablyF, Cl, CF₃, OCF₃ or OCHF₂, in particular F or OCF₃.

[0095] The liquid crystal mixture preferably comprises in addition oneor more compounds selected from the formulae XVII to XIX.

[0096] wherein R⁰, Y¹, Y² and X⁰ have the meanings given above, and thephenylene rings are optionally mono- or polysubstituted with F or Cl.Preferably R⁰ is n-alkyl with 1 to 8 C atoms or alkenyl with 2 to 7 Catoms, X⁰ is F, Cl, CF₃, OCF₃ or OCHF₂, in particular F or Cl, and atleast one phenylene ring is substituted with F.

[0097] Preferred compounds of formula XVII are those of formula XVIIa

[0098] wherein X⁰ is F or Cl, in particular F.

[0099] Preferred compounds of formula XVIII are those wherein Y¹ is Fand X⁰ is F or Cl, in particular F.

[0100] The liquid crystal mixture preferably comprises in addition oneor more compounds selected from the formulae XX to XXIII.

[0101] wherein R³ and R⁴ have independently of each other one of themeanings of R⁰ as defined above and Y¹ is H or F.

[0102] In the compounds of formula XX R³ and R⁴ are preferably alkylwith 1 to 8 C atoms or alkenyl with 2 to 7 C atoms.

[0103] In the compounds of formula XXI, XXII and XXIII R³ and R⁴ arepreferably alkyl or alkoxy with 1 to 8 C atoms.

[0104] Compounds of the formula XXIII, wherein Y¹ is F, are especiallypreferred.

[0105] The invention furthermore relates to an electrooptical display,preferably a display having active matrix addressing and/or a reflectivedisplay. The inventive display is preferably based on the OCB effect, aprojection display and/or an LCoS™ display, characterized in that itcontains, as a dielectric, a liquid-crystalline medium as describedabove.

[0106] Preferably the mixture essentially comprises compounds selectedfrom the group comprising the generic formulae I, IIa, IIb and III toXXIII.

[0107] Preferred embodiments of the the present invention relate tomixtures that contain:

[0108] at least 2%, preferably at least 3%,-most preferably at least 4%,and preferably up to 20%, most preferably up to 15% by weight of one ormore compounds of the formula I.

[0109] at least 10%, preferably at least 15%, and preferably up to 45%,most preferably up to 40% by weight of one or more compounds of theformula IIa.

[0110] at least 3%, preferably at least 5%, and preferably up to 40%,most preferably up to 30% by weight of one or more compounds of theformula IIb.

[0111] at least 10%, preferably at least 15%, and preferably up to 75%,most preferably up to 60% by weight of one or more compounds of theformulae IV,

[0112]  in particular at least 8%, preferably at least 15%, andpreferably up to 50%, most preferably up to 40% by weight of one or morecompounds of the formulae IVb and/or IVc, and

[0113]  in particular at least 1%, preferably at least 3%, andpreferably up to 20%, most preferably up to 15% by weight of one or morecompounds of the formula IVa.

[0114] at least one compound of formula Ia and/or Ib.

[0115] at least one compound of formula IIa, in particular wherein X⁰ isF, especially wherein R⁰ is alkyl with 1 to 8 C-atoms.

[0116] at least one compound of formula IIb, in particular wherein X⁰ isCl, especially wherein R⁰ is alkyl with 1 to 8 C-atoms.

[0117] at least one compound of formula IVb and/or IVc, in particularwherein X⁰ is F, especially wherein R⁰ is alkyl with 1 to 8 C-atoms.

[0118] at least one compound of formula IVa, in particular wherein X⁰ isF, especially wherein R⁰ is alkyl with 1 to 8 C-atoms.

[0119] at least one compound of formula IIIa1, in particular whereinalkenyl is vinyl.

[0120] at least one compound of formula IXa, in particular wherein X⁰ isF.

[0121] one or more compounds of formula XVII1a, in particular wherein X⁰is F, OCF₃ or OCHF₂.

[0122] at least one compound of formula XXIII, in particular wherein Y¹is F.

[0123] The compounds of formula IIa, IIb, and III to XXIII are knownfrom prior art or can be prepared according to known methods.

[0124] By using compounds of formula I it is possible to obtaininventive highly polar liquid crystalline mixtures with at the same timehigh birefringence and high clearing point. In addition, the use ofcompounds of formula IIa leads to high values of the dielectricalanisotropy in the inventive mixtures. Furthermore, liquid crystalmixtures according to the invention, comprising compounds of formulaIIb, exhibit a very high birefringence.

[0125] The liquid-crystal mixture preferably has a nematic phase rangeof at least 75 K, in particular at least 90 K, and a clearing pointabove 70° C., in particular above 80° C., especially above 90° C.

[0126] The dielectric anisotropy is preferably greater than +8, verypreferably at least +10, most preferably equal or greater than +11.

[0127] The mixtures according to the present invention usually are basedon medium polar components having the indicated core structure and othernon-cyano components. Of course, however, such mixtures can alsoadditionally contain known cyano liquid crystalline components,preferably compounds of the formula XXIV

[0128] wherein R⁰, r, Y¹ and Y² have the meanings given above, and

[0129] is trans-1,4-cyclohexylene or 1,4-phenylene, if extremely highvalues for the holding ratio (HR) are not needed, e.g. for TN orSTN-use. The resulting mixtures are important for achieving very broadnematic phase ranges including very low temperatures (outdoor use).

[0130] The mixtures are preferably based on halogenated components ofmedium polarity and/or are essentially free of cyano components.

[0131] The liquid crystal mixtures according to the present inventionare also suitable as liquid crystal media for use in cholesteric liquidcrystal displays, in particular in SSCT (“surface stabilized cholesterictexture”) and PSCT- (“polymer stabilized cholesteric texture”) displays,as described for example in WO 92/19695, U.S. Pat. Nos. 5,384,067,5,453,863, 6,172,720 or 5,661,533. Cholesteric liquid crystal displaystypically comprise a cholesteric liquid crystal medium consisting of anematic component and an optically active component and, compared to TN-and STN displays, exhibit a significantly higher helical twist and showselective reflection of circular polarised light. The reflectionwavelength is given by the product of the pitch of the cholesteric helixand the mean refractive index of the cholesteric liquid crystal medium.

[0132] For this purpose one or more chiral dopants are added to theliquid crystal mixture according to the present invention, wherein thetwisting power and concentration of the dopants are selected such thatthe resulting liquid crystal medium has a cholesteric phase at roomtemperature and a reflection wavelength that lies preferably within thevisible, UV or IR range of the electromagnetic spectrum, in particularwithin the range from 400 to 800 nm. Suitable chiral dopants are knownto the expert and commercially available, like for example cholesterylnonanoate (CN), CB15, R/S-811, R/S-1011, R/S-2011, R/S-3011 or R/S-4011(Merck KGaA, Darmstadt). Particularly suitable are dopants with hightwisting power comprising a chiral sugar group, in particular sorbitolderivatives as disclosed in WO 98/00428. In case two or more dopants areused, they can exhibit the same or opposite twist sense and the same oropposite sign of the linear temperature coefficient of the twist.

[0133] In the components of the formulae IIa, IIb and III to XXIV R⁰,R¹, R³, and R⁴ are preferably a straight-chained alkyl radical of 1 to 8carbon atoms or is straight-chained methoxy alkyl. Preferred alkylgroups are methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl andoctyl, in particular ethyl, propyl, butyl and pentyl. Preferred methoxyalkyl groups are methoxymethyl, methoxyethyl, methoxypropyl,methoxy-butyl, methoxypentyl, methoxyhexyl, methoxyheptyl.

[0134] The term “alkoxy” comprises straight chain and branched alkoxygroups with 1 to 8 C atoms. Straight chain alkoxy groups are preferred.Thus, preferred alkoxy groups are methoxy, ethoxy, propoxy, butoxy,pentoxy, hexoxy, heptoxy and octoxy.

[0135] The term “alkenyl” comprises straight chain and branched alkenylgroups with 2 to 7 C atoms. Straight chain alkenyl groups are preferred.Further preferred alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl,C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6-alkenyl, in particularC₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl.

[0136] Of these, especially preferred alkenyl groups are vinyl,1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl,3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl and 6-heptenyl. Alkenyl groups withup to 5 C atoms are particularly preferred.

[0137] In the foregoing and in the following those compounds containingat least one cyclohexane ring are preferred wherein the cyclohexane ringis trans-substituted.

[0138] The preparation of the mixtures according to the invention iseffected in the conventional manner. In general, the desired amount ofthe components which is used in the smaller amount is dissolved in thecomponents which constitutes the main constituent, preferably atelevated temperature. If this temperature is chosen to be above theclearing point of the main constituent, the completeness of the processof dissolving can be observed particularly easily.

[0139] However, it is also possible to mix solutions of the componentsin a suitable organic solvent, for example acetone, chloroform ormethanol, and to remove the solvent, e.g. by distillation.

[0140] By means of suitable additives the liquid crystal phasesaccording to the invention can be modified in such a way that they canbe used in any hitherto disclosed kind of MLC display.

[0141] In the present application and in the examples below, thestructures of the liquid-crystal compounds are indicated by means ofacronyms, the transformation into chemical formulae taking place inaccordance with Tables A and B below. All radicals C_(n)H_(2n+1),C_(m)H_(2m+1) and C_(r)H_(2r+1) are straight-chain alkyl radicals and—C_(S)H_(2s)— is a straight-chain alkylen group having n, m, r and scarbon atoms respectively. n, m, r and s are integers and areindependently of each other preferably 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11 or 12. The coding in Table B is self-evident. In Table A, only theacronym for the parent structure is indicated. In individual cases, theacronym for the parent structure is followed, separated by a dash, by acode for the substituents R¹, R², L¹ and L² as follows: Code for R¹, R²,L¹, L² R¹ R² L¹ L² nm C_(n)H_(2n+1) C_(m)H_(2m+1) H H nOm C_(n)H_(2n+1)OC_(m)H_(2m+1) H H nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) H H n C_(n)H_(2n+1)CN H H nN.F C_(n)H_(2n+1) CN H F nF C_(n)H_(2n+1) F H H nOFOC_(n)H_(2n+1) F H H nCl C_(n)H_(2n+1) Cl H H nF.F C_(n)H_(2n+1) F H FnF.F.F C_(n)H_(2n+1) F F F nCF₃ C_(n)H_(2n+1) CF₃ H H nOCF₃C_(n)H_(2n+1) OCF₃ H H nOCF₂ C_(n)H_(2n+1) OCHF₂ H H nS C_(n)H_(2n+1)NCS H H rVsN C_(r)H_(2r+1)—CH═CH—C_(s)H_(2s)— CN H H rEsNC_(r)H_(2r+1)—O—C_(s)H_(2s)— CN H H nAm C_(n)H_(2n+1) COOC_(m)H_(2m+1) HH nOCCF₂.F.F C_(n)H_(2n+1) OCH₂CF₂H F F

[0142] Preferred mixture components are found in Tables A and B. TABLE A

PYP PYRP

BCH CBC

CCH CCP

CPTP

CEPTP

ECCP CECP

EPCH PCH

PTP BECH

EBCH CPC

B FET

CGG CGU

CUP

[0143] TABLE B

CBC-nmF

CCGU-n-F

PCH-nOm

FET-nCl

CP-nOCF₃

CCH-nOm

BCH-n.Fm

Inm

CPGU-n-OT

ECCP-nm CCH-n1EM

GGP-n-Cl PGIGI-n-F

PGU-n-F PGIGI-n-Cl

T-nFm CGU-n-F

CCP-nOCF₃.F CGG-n-F

CCP-nOCF₂.F(.F) CCP-nF.F.F.F

CGU-n-OXF CUZU-n-F

CGU-n-O1DT

CCZU-n-F

CC-n-V1 CC-n-V

CCP-nOCF₃

BCH-nF.F.F

CWCZU-n-F

CWCZG-n-F

CCOC-n-m

CGZU-n-F

CUZP-n-F

CGU-1V-F CCG-V-F

CGZP-n-F

CGZP-n-OT

CUZP-n-OT

CCQU-n-F Dec-U-n-F

Nap-U-n-F

CWGZG-n-F

CWCZG-n-OT

CWCZP-n-OT

[0144] TABLE C Suitable dopants are listed in table C. One or more ofthese are generally added to the liquid crystalline medium according tothe invention.

C 15 CB 15

CM 21

R/S 811

CM 44

CM 45 CM 47

CN

R/S-1011

R/S-2011

R/S-3011

R/S-4011

[0145] TABLE D Examples of stabilizers, which may be added to the liquidcrystalline mixtures according to the invention, are listed below.

[0146] Particular preference is given to mixtures according to theinvention which, besides one or more compounds of the formula 1,comprise two, three or more compounds selected from Table B.

[0147] The following abbreviations are used

[0148] Δn denotes the optical anisotropy measured at 20° C. and 589 nm

[0149] n_(e) denotes the extraordinary refractive index at 20° C. and589 nm

[0150] Δε denotes the dielectric anisotropy at 20° C.

[0151] ε_(∥) denotes the dielectric constant in the parallel directionto the molecular axis

[0152] cp denotes the clearing point [° C.]

[0153] γ₁ denotes the rotational viscosity [mPa s]

[0154] V₀ Fredericksz threshold [V]

[0155] THF tetrahydrofurane

[0156] MTB tert.-butyl-methyl-ether

[0157] C=crystalline state, N=nematic phase, Sm=smectic phase andI=isotropic phase. The data between these symbols are the transitiontemperatures.

[0158] Without further elaboration, it is believed that one skilled inthe art can, using the preceding description, utilize the presentinvention to its fullest extent. The following preferred specificembodiments are, therefore, to be construed as merely illustrative, andnot limitative of the remainder of the disclosure in any way whatsoever.

[0159] In the foregoing and in the following examples, all temperaturesare set forth uncorrected in degrees Celsius and, all parts andpercentages are by weight, unless otherwise indicated.

EXAMPLE 1 Synthesis of CPGU-3-OT

[0160] 1.1 Synthesis of the Boronic Acid (3)

[0161] 1.375 mol Mg in 100 ml THF are reacted with 1.25 mol1-brom-3-fluorobenzene (1) in 350 ml THF under the conditions of aGrignard reaction at boiling heat. After the addition of thefluorobenzene, the reaction mixture is refluxed for 30 min. Afterwards1.375 mol trimethylborate (2) in 300 ml THF are added to the reactionmixture dropwise. After one hour 500 ml MTB are added and then 200 mlconcentrated HCl are added dropwise at a maximum of 20° C., whilecooling. The following work up is conventional.

[0162] 1.2 Synthesis of 3-fluoro-4′-(trans-4-propyl-cyclohexyl)-biphenyl(5)

[0163] 0.004 mol bis(triphenylphosphin)-palladium(II)-chloride and 0.004mol hydraziniumhydroxide are added to 0.100 mol sodiummetaborat-octahydrate in 80 ml water. To this mixture 0.200 mol1-bromo-4-(trans-4-propyl-cyclohexyl)benzene (4) in 150 ml THf are addeddropwise. After stirring for 5 min, 0.200 mol of the boronic acid (3) in50 ml THF are added dropwise. The reaction mixture is refluxed for 10hours, cooled to room temperature (20° C.) and worked up conventionally.

[0164] 1.3 Synthesis of the Boronic Acid (7)

[0165] 0.093 mol of the compound 5 in 60 ml THF are added to 0.093 molpotassium-tert-butylate in 100 ml THF at −80° C. dropwise. Furthermore,at −80° C. 0.093 mol butyllithium (15% solution in n-hexane) are addeddropwise. The reaction mixture is stirred for 20 min. Afterwards 0.093mol trimethylborate in 60 ml THF is added dropwise. After allowing thereaction mixture to warm to room temperature (20° C.), 200 ml water areadded and a pH of 2 is achieved with 40 ml half concentrated HCl,followed by a conventional work-up.

[0166] 1.4 Synthesis of CPGU-3-OT (9)

[0167] A solution of 0.040 mol Na₂CO₃ in 30 ml water is added to amixture of 25 mmol of the bromide (8), 25 mmol of the boronic acid (7),80 ml 2-propanole, 0.125 mmol palladium(II)acetate and 0.375 mmoltriphenylphosphine. The reaction mixture is refluxed under stirring for6-12 hours. After cooling to 20° C., the reaction mixture is poured onto400 ml water. The precipitate is filtered by vaccuum and washed withcold water and ethanol. The resulting powder is recrystallized. A NMRand mass spectroscopy analysis yields the expected signals.

[0168] The following compounds are prepared by procedures similar to theprevious example. Compounds where X¹ is Fn-OCHF₂ are comparative. R¹ X¹—CH₃ —F —C₂H₅ —F K 120 N 197.5 I, Δn = 0.2213, Δε = 17.8, γ₁ = 414 mPa ·s —C₃H₇ —F K 113 N 223.8 I, Δn = 0.2316, Δε = 17.2, γ₁ = 790 mPa · s—C₄H₉ —F K 108 N 215.1 I, Δn = 0.2219, Δε = 16.9, γ₁ = 659 mPa · s—C₅H₁₁ —F —C₆H₁₃ —F —C₇H₁₅ —F —CH₃ —OCHF₂ —C₂H₅ —OCHF₂ K 106 N 228.0 I,Δn = 0.2158, Δε = 16.4, γ₁ = 640 mPa · s —C₃H₇ —OCHF₂ —C₄H₉ —OCHF₂ K 54SmA 62 N 247.5 I, Δn = 0.2109, Δε = 15.5, γ₁ = 1,014 mPa · s —C₅H₁₁—OCHF₂ —C₆H₁₃ —OCHF₂ —C₇H₁₅ —OCHF₂ —CH₃ —OCF₃ —C₂H₅ —OCF₃ K 125 N 220.5I, Δn = 0.2090, Δε = 21.9, γ₁ = 572 mPa · s —C₃H₇ —OCF₃ K 94 N 240.0 I,Δn = 0.2165, Δε = 19.7, γ₁ = 802 mPa · s —C₄H₉ —OCF₃ K 76 SmA (57.5) N235.1 I, Δn = 0.2092, Δε = 20.4, γ₁ = 798 mPa · s —C₅H₁₁ —OCF₃ —C₆H₁₃—OCF₃ —C₇H₁₅ —OCF₃

EXAMPLE 2

[0169] A liquid crystal mixture is prepared that comprises PGIGI-3-F11.00% cp 101.0° C. PGU-2-F 10.00% n_(e) 1.6912 PGU-3-F 10.00% Δn 0.1809PGU-5-F  9.00% ε_(∥) 19.1 BCH—2F.F  9.00% Δε 14.5 BCH—3F.F  9.00% V₀0.95 V BCH—5F.F  9.00% BCH—5F.F.F  4.00% CGU-3-F  5.00% CCGU-3-F  9.00%CBC—53F  4.00% CPGU-3-OT 11.00%

EXAMPLE 3

[0170] A liquid crystal mixture is prepared that comprises PGIGI-3-F11.00% cp 99.0° C. PGU-2-F  8.00% n_(e) 1.6872 PGU-3-F 10.00% Δn 0.1763PGU-5-F  8.00% BCH—2F.F  9.00% BCH—3F.F 10.00% BCH—5F.F 10.00%BCH—5F.F.F  5.00% CGU-3-F  6.00% CCGU-3-F 10.00% CBC—53F  3.00%CPGU-3-OT 10.00%

EXAMPLE 4

[0171] A liquid crystal mixture is prepared that comprises PGIGI-3-F11.00% cp 103.0° C. PGU-2-F 10.00% n_(e) 1.6930 PGU-3-F 12.00% Δn 0.1827PGU-5-F  9.00% BCH—2F.F  7.00% BCH—3F.F  8.00% BGH—5F.F  8.00%BCH—5F.F.F 11.00% CCGU-3-F 10.00% CBC—53F  2.00% CBC—33F  3.00%CPGU-3-OT  9.00%

EXAMPLE 5

[0172] A liquid crystal mixture is prepared that comprises PGIGI-3-F11.00% cp 102.0° C. PGU-2-F 10.00% n_(e) 1.6941 PGU-3-F 12.00% Δn 0.1836PGU-5-F  9.00% BCH—2F.F  8.00% BCH—3F.F  8.00% BCH—5F.F  9.00%BCH—5F.F.F  4.00% CGU-3-F  4.00% CCGU-3-F 10.00% CBC—53F  5.00%CPGU-3-OT 10.00%

EXAMPLE 6

[0173] A liquid crystal mixture is prepared that comprises GGP-3-CL 8.00% cp 102.0° C. GGP-5-CL 20.00% n_(e) 1.7137 PGIGI-3-F 11.00% Δn0.1992 BCH—3F.F  5.00% ε_(∥) 16.7 BCH—5F.F  5.00% Δε 12.0 BCH—3F.F.F 5.00% V₀ 1.13 V BCH—5F.F.F  5.00% CCG-V-F 12.00% PGU-2-F  8.00% PGU-3-F11.00% CBC—53F  5.00% CPGU-3-OT  5.00%

COMPARISON EXAMPLE

[0174] A liquid crystal mixture is prepared that comprises GGP-5-CL16.00% cp 102.0° C. BCH—2F.F 11.00% n_(e) 1.6692 BCH—3F.F 11.00% Δn0.1610 BCH—5F.F  6.00% ε_(∥) 15.3 CGU-2-F  9.00% Δε 10.9 CGU-3-F  9.00%V₀ 1.14 V CGU-5-F  8.00% BCH—3F.F.F  8.00% CCGU-3-F  7.00% BCH-32 10.00%CBC-33  3.00% CBC-53  2.00%

[0175] and exhibits a significantly lower optical anisotropy than themixtures of examples 1 to 5 comprising a compound of formula I

[0176] (CPGU-n-OT).

[0177] The entire disclosures of all applications, patents andpublications, cited herein and of corresponding European application No.01126409.0, filed Nov. 7, 2001 are incorporated by reference herein.

[0178] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

[0179] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A liquid crystalline compound of formula I

wherein X¹ is OCF₃ and R¹ is alkyl with 1 to 20 C atoms.
 2. A liquidcrystalline medium containing at least two liquid-crystalline compounds,at least one of which is a compound according to claim
 1. 3. Aliquid-crystalline medium based on a mixture of polar compounds ofpositive dielectric anisotropy, comprising at least one compound offormula I

wherein X¹ is OCF₃ and R¹ is alkyl with 1 to 20 C atoms.
 4. A mediumaccording to claim 3, having an optical anisotropy Δn≧0.16.
 5. A mediumaccording to claim 3 additionally comprising at least one compound offormula IIa

wherein R⁰ is alkyl, alkoxy, fluoroalkyl, alkenyl or oxaalkenyl with upto 9 C atoms and X⁰ is F, Cl, CF₃, OCF₃, OCHF₂, fluoroalkyl orfluoroalkoxy with up to 7 carbon atoms.
 6. A medium according to claim3, additionally comprising at least one compound of formula IIb

wherein R⁰ is alkyl, alkoxy, fluoroalkyl, alkenyl or oxaalkenyl with upto 9 C atoms, X⁰ is F, Cl, CF₃, OCF₃, OCHF₂, fluoroalkyl or fluoroalkoxywith up to 7 carbon atoms, and Y¹ and Y² are each independently H or F.7 A medium according to claim 3, additionally comprising at least onecompound of formula IV

wherein R⁰ is alkyl, alkoxy, fluoroalkyl, alkenyl or oxaalkenyl with upto 9 C atoms, Z¹ is CF₂O, C₂F₄ or a single bond, X⁰ is F, Cl, CF₃, OCF₃,OCHF₂, fluoroalkyl or fluoroalkoxy with up to 7 carbon atoms, Y¹ to Y⁴are each independently H or F, and r is 0 or
 1. 8. A medium according toclaim 3, additionally comprising at least one compound of formulae III,IX, XVII or XXIII

wherein R⁰, R³, R⁴ are independently of each other alkyl, alkoxy,fluoroalkyl, alkenyl or oxaalkenyl with up to 9 C atoms, X⁰ is F, Cl,CF₃, OCF₃, OCHF₂, fluoroalkyl or fluoroalkoxy with up to 7 carbon atoms,Y¹ to Y³ are each independently H or F, and r is 0 or
 1. 9. A mediumaccording to claim 3, comprising 3 to 20% by weight of at least onecompound of formula
 1. 10. A medium according to claim 5, comprising 10to 40% by weight of at least one compound of formula IIa.
 11. A mediumaccording to claim 6, comprising 5 to 30% by weight of at least onecompound of formula IIb.
 12. A cholesteric liquid crystalline mediumcomprising a nematic component which is a liquid crystalline mediumaccording to claim 3 and an optically active component which is at leastone chiral dopant.
 13. An electrooptical liquid-crystal display,comprising a liquid-crystalline medium according to claim
 3. 14. Anelectrooptical liquid-crystal display according to claim 13, which is anactive matrix display.
 15. An electrooptical liquid-crystal displayaccording to claim 13, which is a reflective display.
 16. Anelectrooptical liquid-crystal display according to claim 13, which is aprojection, LCoS™ or OCB mode display.
 17. A cholesteric liquid crystaldisplay comprising a cholesteric liquid crystalline medium according toclaim
 12. 18. A cholesteric liquid crystal display according to claim17, which is a SSCT- or PSCT-display.